Research in progress concerns an investigation of autoreactive antibodies induced by hyperimmunization of rabbits with bacterial vaccines. Rabbits hyperimmunized with group C streptococcal vaccine generate, in addition to anti-streptococcal carbohydrate antibody, cold agglutinin and rheumatoid factor antibodies. The cold agglutinin antibodies appear in both the IgM and IgG classes, and on the basis of ligand reactivity, appear specific for the group C carbohydrate. However, experiments are in progress to determine whether the state of hyperimmunization has induced an erythrocyte-specific response which is cross-reactive with group C carbohydrate. Rabbit rheumatoid factor, both IgM and IgG, have been purified from hyperimmune antistreptococcal serum and have been examined for complement activation ability and specificity for rabbit IgG. Both rabbit IgM and IgG rheumatoid factor activate complement, but relative to other rabbit antibodies, do not appear as efficient with regard to complement interaction. In addition, although reactive with human IgG, rabbit rheumatoid factor appears specific for the Fc region of rabbit IgG, particularly if the Fc region interheavy-chain disulfide is cleaved.